1. Field of the Invention
The present invention relates to prosthetic devices particularly shoulder prostheses and, more particularly, to a method and apparatus for replication of angular position of a humeral head of a shoulder prosthesis.
2. Background Information
The state of the prosthetic shoulder market has progressed such that a surgeon generally approaches shoulder replacement surgery according to one of two strategies. One strategy is to perform the shoulder replacement surgery in accordance with a manufacturer's shoulder prosthesis or shoulder prosthesis product line. Particularly, a surgeon is provided with instrumentation and technique guidelines for the particular shoulder prosthesis or prosthesis line. The guidelines and/or instrumentation direct or dictate the angle of humeral head resection for the implant (prosthesis). This angle is in relation to the humeral intramedullary (IM) canal and is designed to match an optimum set of angles already present in the design of the prosthesis.
Another approach is to perform the shoulder replacement surgery in accordance with the patient's anatomy. Particularly, the humeral head is resected according to angles perceived to be “anatomic” in the opinion of the surgeon, not according to angles already present in the prosthesis itself. With this approach, the prosthesis is designed so that its configuration is intraoperatively adjustable. This allows the prosthesis to be adjustable in situ so that it can match the bony preparation.
Even with respect to these two divergent manners of surgical strategy, a common problem in shoulder surgery is matching the humeral resection angle across the articular margin to the predetermined angle designed into the prosthesis. This angle may include the angle between a prosthetic collar and the diaphyseal section of the stem. In the case of a collarless stem, the angle may inscribe the difference between the longitudinal axis of the stem and the inferior surface of the prosthetic head. It is considered optimal for fixation and biomechanics if the resected angle and the angle of the prosthesis are identical, thereby allowing intimate contact between the superior surface of resected bone and the inferior surface of the implant.
Moreover, the angular version in which the prosthesis is implanted will have a significant impact on the biomechanics of the prosthetic joint. Currently, most shoulder prosthesis systems on the market dictate the varus/valgus angle of the bone cut. This strategy does not allow the surgeon to intraoperatively match the implant to the patient's biomechanics after the prosthesis has been trialed, much less implanted. There are two known products currently marketed that attempt to resolve at least one of the above-noted issues. First, the Tornier-Aequalis system provides a modular junction within the metaphyseal region of the stem which allows a small block between the stem and humeral head to be interchanged. This block is available in multiple angles, thus allowing the surgeon to select the block that best fits the bony anatomy as resected. This system, however, has two primary weaknesses. First, the use of modular blocks obviously forces the design to only allow angular adjustments in finite increments. Second, the need to adjust the angle through modular blocks forces the surgeon to remove the stem, change out a component, and reset the stem.
A second product currently marketed provides a humeral head that is infinitely adjustable in varus/valgus and anterior/posterior angles relative to the stem portion of the prosthesis. This is accomplished through a spherical shaped protrusion on the superior surface of the stem that fits into a spherical recess in the humeral head. These mating surfaces allow the head to be articulated about the stem, thus allowing adjustable positioning of the head. The head can be locked in a position relative to the stem. This solution provides the ability to adjust the neck-shaft angle as well as the version through flexibility in the anterior/posterior angle. The locking mechanism, however, is sub-optimal since it requires the turning of a locking screw that has its head facing lateral and inferior, for which there is no access once the stem has been cemented. This eliminates the ability to adjust head position on the fly, and forces a total revision if articular surfaces ever need to be revised. Lastly, the protrusion on the humeral stem even when the humeral head is not in place limits the surgeon's access to the glenoid in preparation for a glenoid replacement.
It should be appreciated that it is desired to have a prosthesis that is intraoperatively adjustable so that it can match the bony preparation. One such prosthesis that has attempted to provide this design feature provides a complicated and cumbersome jig to permit replication of the head angular position between the trial prosthesis and the final implant. One problem with this jig is that the head position is taken directly from the long axis of the humeral stem. Thus, the trial and implant stems are required to adjust and replicate this position. This means that the system has a large number of components to handle, and the position cannot be adjusted with the stem in vivo. Another problem is that the jig itself is a quite complicated table-top device. The jig includes an adjustment gage and a triplanar disc that must be manipulated to effect replication of the head orientation between trial and final implant.
With a shoulder prosthesis that allows a surgeon to adjust the angular position of the humeral head (i.e. adjust the articular surface of the humeral head relative to the humeral stem/component and/or bone) a method must be available for trialing the prosthesis. When the trial prosthesis is implanted, several adjustments can be made to set the angular position of the prosthetic head relative to the humeral stem. A means must be available for transferring the settings obtained during the trialing process to the final implant.
What is thus needed in view of the above is a better method for trialing a shoulder prosthesis, and more particularly for replicating the orientation of the trial implant components in the final implant. There is also a need for a replication system that is quick and easy to operate, as well as for a system that can be utilized with implants utilizing locking taper fixation.